What is the relation between multi-agent learning and reinforcement learning?
I think there is an intersection. There are problems that are in reinforcement learning and in learning in multi-agent systems. There are problems in reinforcement learning, but not exactly in multi-agent systems. And there is learning in multi-agent systems that is not through reinforcement learning. For sort you can say: multi-agent reinforcement learning. I recommend take a look at these references:

Bu, Lucian, Robert Babu, and Bart De Schutter. &quot;A comprehensive survey of multiagent reinforcement learning.&quot; IEEE Transactions on Systems, Man, and Cybernetics, Part C (Applications and Reviews) 38.2 (2008): 156-172.
Busoniu, Lucian, Robert Babuska, and Bart De Schutter. &quot;Multi-agent reinforcement learning: A survey.&quot; 2006 9th International Conference on Control, Automation, Robotics and Vision. IEEE, 2006.